[PDF] Fundamentals of Foods, Nutrition and Diet Therapy, 5th Edition

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Foreword

FOODS AND NUTRITION are essential for maintaining good health and to prevent disease. Altho ugh food occupies the first position in the hierarchical needs of man, ignor ance of many basic facts relating

to foods and nutrition is still widespread. Consequently, various nutritional disorders for which there

are simple remedies such as blindness caused by vitamin A deficiency per sist. Good nutrition is a

function of both economics and education. In our country, the most serious form of nutritional disorder

is undernutrition arising from inadequate purchasing power. However, even when purchasing power is adequate to access balanced nutrition, various forms of imbalance in die tary intake occur due to lack of knowledge. The culinary art of which we are justifiably proud, has placed organolep tic considerations as the sole criteria of excellence. The various opportunities for accomplishing balanced nutrition through synergistic interaction within the components of diet have not received the consideration they deserve. I am, therefore, happy that Dr. Sumati R. Mudambi and Dr. M.V. Rajagopal have taken the trouble of

revising their excellent book Fundamentals of Foods and Nutrition and added Diet Therapy in the fifth

edition of the book. The book covers all the important areas which ought to be taken into consideration while promoting good nutrition in health and disease. The knowledge of foods and nutrition is essential for teachers, health p rofessionals, students of home sciences and health sciences and civil society organizations engage d in fighting endemic and hidden hunger. The revised edition with the addition of diet therapy will be a valuable book for the use of dietetics and nursing students. It will also be a valuable guide and reference boo k for physicians and other health professionals. The book of Dr. Mudambi and Dr. Rajagopal will make an important contribution to accelerating the pace of achieving good nutrition in health and disease.

The publication of the revised version is timely.

Dr. M.S. Swaminathan

Unesco Cousteau Chair in Ecotechnology & Chairman,

M.S. Swaminathan Research Foundation,

Chennai 600113.

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Preface to the Fifth Edition

WE ARE HAPPY to present the 5th edition of Fundamentals of Foods, Nutrition and Diet Therapy.

This book is an introduction to Foods, Nutrition and Diet Therapy. The science courses studied at high

school level are sufficient background for this course. This book has consistently been used by students studying the first cour se in Food Science and Nutrition. In several universities diet therapy topics have been added i n the curricula of this course. Therefore diet therapy has been added in this revision. We hope it meets the altered needs of our readers. The revised book is intended to meet the needs of students in a number o f faculties in which Foods, Nutrition and Diet Therapy is a major, elective or ancillary subject. These include Home Science, Nursing, Medical and Paramedical, Agriculture, Education, Arts, Social Sciences, Community Health, Pediatrics, Environmental Health and Allied areas. The contents of this edition have been revised in view of the recent dev elopments in the area,

logical sequence of subject matter, changes in the syllabi of universities and the feedback from teachers

and students.

The book covers the following aspects:

I. Introduction to the Study of Nutrition (Chapter 1 to 3)

II. The Nutrients and Energy (Chapters 4 to 13)

III.Meal Planning and Management (Chapters 14 to 17)

IV. Foods (Chapters 18 to 22)

V. Diet Therapy (Chapters 23 t0 36)

As in earlier editions, this book covers the principles of nutrition and food science and discusses the application of this knowledge in practice. In addition, in this edit ion it covers the principles of diet therapy, the changes in body that occur due to disease, the altered needs of th e body due to disease and how to meet these through modification of the normal diet. Figures and t ables have been used to help visualise practical information. The contents of the book have been deve loped out of continuous study and teaching of these subjects and use of the knowledge in consultation with patients and use in practical situations. Each chapter opens with an outline of its contents. Any abbreviations us ed are explained, and all terms, which need explanation are included in the Glossary at the end of the book. In the Appendices, Food Exchange Lists, Recommended Dietary Allowances f or Indians and Food Composition Tables have been included. In addition, Table of Indian names of foods, height and weight of girls and boys from birth to eighteen years and other useful i nformation is given. viiiviiiviiiviiiviiiFundamentals of Foods, Nutrition and Diet Therapy We hope this edition helps towards making the goal of good nutrition for all, in health and disease, a reality. Any suggestions for improvement of the book in later edition are very we lcome, for it indicates a shared interest in the study, development and use of this fascinating subject. We are grateful to the staff of New Age International (Private) Limited for their contribution to the development and completion of the fifth edition.

July 2006

Sumati R. Mudambi

M.V. Rajagopal

THERE HAS been a long felt need for a book on the fundamentals of foods and nutri tion for the students preparing for the Higher Secondary School of Certificate Examin ations of both the Central and State Boards of Education. Neither do the students of nutrition in t he first year of Home Science colleges have any compact Indian book on this subject. Most of the colle ges usually recommend American textbooks because of the absence of any suitable Indian book in this area. The American books, however, discuss the subject with reference to the needs of an economy different from ours with the result that students do not learn anything about the applicatio ns of nutrition to Indian conditions. This book covers the subjects of applications of the findings of the foo d science and nutrition to Indian conditions and at the same time discusses in simple language all the basic principles of foods and nutrition. Figures and tables have been used profusely to bring home to the beginner all aspects of foods and nutrition. For the more inquisitive and interested student ref erences to advanced books are given in the bibliography. Through painstaking research, knowledge has been acquired about the nu trition needs of people. It is the aim of this book to motivate teachers and stu dents to make use of this knowledge and bring about a change in the health and welfare of our peo ple. (i) Basic Nutrition (Chapters 1 and 2). (ii)Food Selection and Preparation (Chapters 3, 4 and 5) (iii)Meal Planning (Chapters 6, 7 and 8) (iv)Disorders of Malnutrition (Chapter 9) (v) Food Preservation (Chapters 10 and 11) (vi)Safeguarding the Food Supply (Chapters 12, 13 and 14) It is hoped that this book will help our readers to understand:

1. Functions of foods, which supply our nutritional needs.

2. How to meet human need of nutrients in terms of available foods.

3. Prices are guides of supply and demand and not of their nutritive value.

4. Techniques of preparation which help us meet our needs in an enjoyable ma

nner.

5. Meal planning as a tool in meeting nutritional needs of the family throu

gh acceptable enjoyable meals.

6. Preservation as an aid to improved food availability.

7. Safeguarding the supply through proper selection, careful storage and pr

eparation.

8. One's responsibilities as a consumer.

The difficulties encountered by foods and nutrition teachers, due to lac k of an appropriate Indian text, were brought to our notice by the teachers, who attended the NCERT Summer Workshops conducted by one of us (SRM) at the Lady Irwin College in 1978. It pro mpted us to complete this book, which we had started writing, on a priority basis.

Preface to the First Edition

xxxxxFundamentals of Foods, Nutrition and Diet Therapy Great care has been taken to ensure that no mistakes have crept into the book. However, if there are any which have escaped our scrutiny, we would welcome corrections from our readers. We are grateful to the Director, National Institute of Nutrition for supplying the figurs 9.2, 9.3,

9.4, 9.5, 9.6, and 9.7, to Messrs Voltas Limited for figures 3.1, 10.2, 10.3, 10.4, 12.4 and 14.1 and to

Messrs Balsara & Company for figure 12.2.

We are grateful to late Dr. Leela Phadnis, Professor of Foods & Nutrition, Vanasthali Vidyapeeth,

Vanasthali, Rajasthan, Dr. S.S. Ajagaonkar, former President, All-India Institute of Diabetes, Bombay

and to late Dr. V. Nagarajan, Assistant Director-General, Indian Council of Agricultural

Research, New

Delhi, for their careful appraisal of the manuscript and valuable sugges tions. We sincerely appreciate the assistance of our sons Anand and Shyam, who c orrected typescripts and prepared the glossary for us.

BombaySRM

September, 1982

MVR

Contents

Forewordv

Preface to the Fifth Edition vii

Preface to the First Edition ix

Part I: Introduction to the Study of Nutrition

Chapter 1 Foods, Nutrition and Health ........................................................................

................3

Chapter 2 Digestion, Absorption and Utilisation of Nutrients................................................... 11

Chapter 3 The Recommended Dietary Allowances for Nutrients ............................................26

Part II: The Nutrients and Energy

Chapter 4Carbohydrates........................................................................ ...................................37

Chapter 5 Fats and Other Lipids........................................................................

........................46

Chapter 6 Proteins and Amino Acids ........................................................................

................58 Chapter 7 Energy Metabolism........................................................................ ...........................70

Chapter 8Fat-soluble Vitamins........................................................................

..........................87

Chapter 9 Water-Soluble Vitamins........................................................................

.....................99

Chapter 10 Major and Trace Minerals........................................................................

............... 115

Chapter 11 Water, Fluids, Electrolytes and Acid-base Balance.................................................128

Chapter 12 Nutrition for Fitness,

Athletics and Sports.............................................................136

Chapter 13 Disorders of Nutrition........................................................................

.....................141

Part III: Meal Planning and Management

Chapter 14 Food Guides for Selecting an Adequate Diet..........................................................159

Chapter 15 Meal Planning for the Family........................................................................

..........175

Chapter 16 Meal Planning for Various Age Groups ..................................................................185

Chapter 17 Indian Meal Patterns - Vegetarian & Non-Vegetarian.............................................205

xiixiixiixiixiiFundamentals of Foods, Nutrition and Diet Therapy

Part IV: Foods

Chapter 18 Food Selection, Purchase

and Storage ...................................................................213 Chapter 19 Food Preparation........................................................................ .............................226

Chapter 20 Effect of Preparation on Food Components ..........................................................235

Chapter 21 Factors Affecting Food Acceptance ......................................................................241

Chapter 22 Food Sanitation and Hygiene ........................................................................

..........246

Part V: Diet Therapy

Chapter 23 Adaptation of Normal Diet for Changing Needs ....................................................257

Chapter 24 Principles of Diet Therapy and Therapeutic Nutrition ...........................................261

Chapter 25 Nutrition in Infections, Fever

and Lung Diseases..................................................275

Chapter 26 Nutrition in Diseases of the Gastrointestinal Tract ................................................281

Chapter 27 Nutrition in Diseases of Liver, Gallbladder and Pancreas ......................................287

Chapter 28 Nutrition in Diabetes Mellitus........................................................................

.........293

Chapter 29 Nutrition in Cardiovascular Diseases......................................................................301

Chapter 30Diet and Nutrition in Kidney Diseases ....................................................................314

Chapter 31Nutrition in Cancer ........................................................................

.........................325 Chapter 32Nutrition in Immune System Dysfunction, AIDS and Allergy ..............................329

Chapter 33Nutrition Support in Metabolic Disorders ..............................................................336

Chapter 34Nutrition in Stress, Burns and Surgery ..................................................................343

Chapter 35Nutrition: Addictive Behaviours in Anorexia Nervosa, Bulimia and Alcohol ism ....350

Chapter 36Nutrient Drug Interaction ........................................................................

...............355 Appendices ........................................................................ ........................................................363 Index........................................................................ ........................................................399

PART - I

Introduction to the Study of Nutrition

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CCCCCHAPTERHAPTERHAPTERHAPTERHAPTER 1 1 1 1 1

Foods, Nutrition and Health

Definitions

Food

Nutrition

Adequate Nutrition

Nutritional Status

Malnutrition

Nutritional Care

Health

Functions of Food

Physiological, Social and Psychological Functions

Functions of Nutrients

Carbohydrates, Fats, Proteins, Vitamins, Minerals and Water

Food Composition

Food Exchange Lists

Nutrient Density

How Will you be a Responsible Nutrition Student?

Study Questions

FOOD HAS been a basic part of our existence. Through the centuries we have acqui red a wealth of information about the use of food to ensure growth of children and yo uth, to maintain good health through life, and to meet special needs of pregnancy and lactation and t o use it to recover from illness. When you study food composition you will know the nutritional contributi on of foods. You may have been told that certain foods are very important for maintaining goo d health, while others are harmful. As you study the science of Foods and Nutrition, you will need to examine the ideas you have about foods very carefully and accept or reject these in the light of th e knowledge you will acquire. Whatever you learn in this area should be used and applied in your perso nal life. A large part of our food heritage is scientifically beneficial and needs to be retained; some aspects may need to be modified in the view of the changes in our lifestyle. Food is an important topic of conversations, articles in newspapers and magazines, as also of advertisements. Some of this information may be correct, but a large par t of it may not be. As you learn

44444Fundamentals of Foods, Nutrition and Diet Therapy

this subject, you will be able to spread the knowledge gained to those a round you, so that they discard false ideas about food, which interfere with their food selection and af fect their health. Food, nutrition and health are intimately connected aspects of our life. Let us start our study by defining these and related terms.

Definitions

Food is that which nourishes the body. Food may also be defined as anything eaten or drunk,

which meets the needs for energy, building, regulation and protection of the body. In short, food is the

raw material from which our bodies are made. Intake of the right kinds a nd amounts of food can ensure good nutrition and health, which may be evident in our appearance , efficiency and emotional well-being (Figure 1.1). Nutrition has been defined as food at work in the body. Nutrition includes everything that happens to food from the time it is eaten until it is used for various functions in the body. Nutrients are components of food that are needed by the body in adequate amounts in order to grow , reproduce and lead a normal,

healthy life. Nutrients include water, proteins, fats, carbohydrates, minerals and vitamins. There are

several nutrients in each of the groups: proteins, fats, carbohydrates, minerals and vitamins; hence the plural form of these words has been used. Thus there are over 40 essenti al nutrients supplied by food, which are used to produce literally thousands of substances necessary fo r life and physical fitness. The study of the science of nutrition deals with what nutrients we need, how much we need, why we need these and where we can get them. Nutrition is the result of the kinds of foods supplied to the body and how the body uses the food supplied. Adequate, optimum and good nutrition are expressions used to indicate that the supply of the essential nutrients is correct in amount and proportion. It also implies that the utilisation of such nutrients in the body is such that the highest level of physical and men tal health is maintained throughout the life-cycle. Figure 1.1: A well-nourished child engrossed in play.

Foods, Nutrition and Health55555

Nutritional status is the state of our body as a result of the foods consumed and their us e by the body. Nutritional status can be good, fair or poor. The characteristics of good nutritional status are an alert, good natured personality, a well developed body, with normal weight for height, well developed and firm muscles, health y skin, reddish pink colour of eyelids and membranes of mouth, good layer of subcutaneou s fat, clear eyes, smooth and glossy hair, good appetite and excellent general health. General good health is evi dent by stamina for work, regular meal times, sound regular sleep, normal elimination an d resistance to disease. Poor nutritional status is evidenced by a listless, apathetic or irritable personality, undersized poorly developed body, abnormal body weight (too thin or fat and flabby body), muscles smal l and flabby, pale or sallow skin, too little or too much subcutaneous fat, dull or reddened eyes, lustreless and rough hair, poor appetite, lack of vigour and endurance for work and susceptibilit y to infections. Poor nutritional status may be the result of poor food selection, irregularit y in schedule of meals, work, sleep and elimination. The WHO (World Health Organization) has defined health as the 'state of complete physical, mental and social well-being and not merely the absence of disease or in firmity'. Malnutrition means an undesirable kind of nutrition leading to ill-health. It result s from a lack, excess or imbalance of nutrients in the diet. It includes undernutrition and overnutrition. Undernutrition is a state of an insufficient supply of essential nutrients. Malnutrition can be primarily be due to insufficient supply of one or mo re essential nutrients; or it can be secondary, which means it results from an error in metabolism, interaction betwee n nutrients or nutrients and drugs used in treatment. Overnutrition refers to an excessive intake of one or more nutrients, wh ich creates a stress in the bodily function. Diet refers to whatever you eat and drink each day. Thus it includes the normal diet you consume and the diet people consume in groups (hostel diet). Diet may also be modified and used for ill persons as part of their therapy (therapeutic diets). Nutritional care is the use of nutritional knowledge in planning meals and the preparati on of these meals in an acceptable and attractive manner to feed people. It in volves assessment of the exiting meal patterns and improving these in an acceptable manner. While the nutritional plan may be general for a group of people, the actual execution is individualized to suit th e person's needs and background. Thus one has to use a lot of ingenuity to succeed in making nutritional care effective in practical terms. Health the word health refers to the condition of the body, good health not only implies freedom from disease, but physical, mental and emotional fitness as well.

Functions of Food

Physiological functions of food. The first function of the body is to provide energy. The body needs energy to sustain the involuntary processes essential for continua nce of life, to carry out professional, household and recreational activities, to convert food ing ested into usable nutrients in the

body, to grow and to keep warm. The energy needed is supplied by the oxidation of the foods consumed.

The foods we eat become a part of us. Thus one of the most important fun ctions of food is building the body. A newborn baby weighing 2.7-3.2 kg can grow to its potential adult size of 50-60 kg if the right kinds and amounts of food are eaten from birth to adulth ood. The food eaten each day helps to maintain the structure of the adult body, and to replace worn out cells of the body.

66666Fundamentals of Foods, Nutrition and Diet Therapy

The third function of food is to regulate activities of the body. It includes regulation of such varied activities as: • Beating of the heart • Maintenance of the body temperature • Muscle contraction • Control of water balance • Clotting of blood • Removal of waste products from the body The fourth function of food is to improve our body's resistance to disease. The Social Functions of Food. Food has always been a central part of our social existence. It

has been a part of our community, social, cultural and religious life. Special foods are distributed as

a benediction or prasad in the religious functions in homes, temples and churches. Feasts are g iven at

specific stages of life such as birth, naming ceremony, birthdays, marriages, etc. Most of the religious

festivals also call for feasts and feeding of specific segments of the p opulation. Certain menus are associated with most of these feasts in each region. Food has been used as an expression of love, friendship and social accep tance. It is also used as

a symbol of happiness at certain events in life, for example, pedhas are distributed to announce success

in examinations, or the birth of a baby; laddus are associated with the celebration of Deepavali and

marriages, cakes are associated with Christmas and birthdays and tilgul with sankranti the festival of

friendship. As food is an integral part of our social existence, this function is im portant in daily life. Refreshments served at get-togethers or meetings create a relaxed atmosp here. The menu for such get-together should bring the people together, rather than divide them. This basic aspect should be considered in planning menus for such occasions (Figure 1.2).

Figure 1.2: Functions of food.

Growth and Renewal

RegulationEnergyLove and Security

FoodParty

Foods, Nutrition and Health77777

The Psychological Functions of Food. In addition to satisfying physical and social needs, food must satisfy certain emotional needs. These includes a sense of security , love and attention. Thus familiar foods make us feel secure. Anticipating needs and fulfilling th ese are expressions of love and attention. These sentiments are the basis of the normal attachment to th e mother's cooking. Sharing of food is a token of friendship and acceptance. In a friendly g athering we try unfamiliar foods and thus enlarge our food experiences. It must be noted that even a nutritionally balanced meal may not be satisfying to the individual, if the foods included are unfam iliar or distasteful to him/her. With time and repeated experience, strange foods become familiar and new tastes are formed. These aspects are important in food acceptance and must be considered in planning meals, which are not only nutritionally adequate, but also enjoyable for the group fo r whom they are intended.

Functions of Nutrients

The foods which we use daily include rice, wheat, dal, vegetables, fruit s, milk, eggs, fish, meat,

sugar, butter, oils, etc. These different foods are made up of a number of chemical components called

nutrients. These are classified according to their chemical composition. Each nutrient class has its own function, but the various nutrients must act in unison for effective action. The nutrients found in foods are - carbohydrates, proteins, f ats, minerals, vitamins and water. Fibre is also an essential component of our diet. The functions of nutri ents are given below.

Carbohydrates:

Starch found in cereals and sugar in sugarcane and fruits are examples of carbohydrates in foods. The chief function of carbohydrates is to provid e energy needed by our body. Those not used immediately for this purpose are stored as glycogen or co nverted to fat and stored, to be mobilised for energy supply when needed. Fats: Oils found in seeds, butter from milk, and lard from meat, are examples of fats found in

foods. Fats are concentrated sources of energy, carriers of fat soluble vitamins and a source of essential

fatty acids. If excess fats are taken in the diet, these are stored as f at reserves in the body. Energy taken in excess of body needs, is stored as fat in the body. Proteins: Casein from milk, albumin in egg, globulins in legumes and gluten in wh eat, are examples of proteins occurring in foods. The main function of protein is the buil ding of new tissues and maintaining and repair of those already built. Synthesis of regulatory and protectiv e substances such as enzymes, hormones and antibodies is also a function of food proteins. About 10 pe r cent of the total energy is supplied by proteins in the diet. Protein, when taken in excess of the b ody's need, is converted to carbohydrates and fats and is stored in the body. Minerals: The minerals calcium, phosphorus, iron, iodine, sodium, potassium and o thers are found in various foods in combination with organic and inorganic compoun ds. Minerals are necessary for body-building, for building of bones, teeth and structural parts of soft tissues. They also play a role

in regulation of processes in the body, e.g., muscle contraction, clotting of blood, nerve stimuli, etc.

Vitamins: Fat-soluble vitamins A, D, E and K and also water-soluble vitamins C an d B group are found in foods. These are needed for growth, normal function of the body and normal body processes. Water: We get water in foods we eat and a major part from the water we drink as such and as beverages. Water is an essential part of our body structure and it accounts for abou t 60 per cent of our body weight. Water is essential for the utilisation of food material in the body and a lso for elimination of food waste. It is a regulator of body processes such as maintenance of b ody temperature.

88888Fundamentals of Foods, Nutrition and Diet Therapy

All individuals need the same nutrients for the same body function. The only variation is in the

amounts of each nutrient required according to age, size, activity, etc. For example, all persons need

energy for work, but a man, who carries loads may need more energy than a man, who works in an office at a desk job. As you know, we get the nutrients from the foods and the beverages we consume. Most foods contain the nutrients in varying amounts. Let us understand the nutrient composition of the foods we use everyday.

Food Composition

Most food contain more than one nutrient. The nutrient contents of foods have been determined by analysing these in the laboratory. The composition of over 650 Indian foods has been determined. Of these, the nutritive value of about 160 foods is presented in the App endix F. The food composition tables give the concentration of nutrients in 100 g of the edible portion (E.P.) of the food. Therefore it is important to know how much of the food purchased is edible. In

some foods, such as milk, butter, sugar, the edible portion is 100 per cent. In fruits and vegetables, it

varies from 65 per cent in bananas to 98 per cent in tomatoes. The values for nutrients given in food composition tables are averages o f the results obtained by analysing a large number of samples of each food. Therefore the figures in such tables give a fairly good idea of the composition of each food. Foods are grouped in the food value tables, on the basis of the plant pa rt from which the food is derived, for example, seeds, roots, leaves, fruits, etc. Animal foods ar e grouped on the basis of species and the product used. It is interesting to note that there are inherent similarities in the co mposition of foods in each group. In Table 1.1, the composition of various foods has been presented to illustr ate this point. For example, the protein content of cereals varies from 7 to 12, and that of dals and legumes from 17 to 25 per cent. This information has important applications in practical usage of tables. It is possible to predict the overall nutrient content of combinations used, if we know th e amounts of individual foods used. If the composition of a particular food is not found in the tables , you can roughly predict its nutrient contribution, by knowing the group to which it belongs. You may observe from Table 1.1, that cereals and dals do not contain vitamins A and C. Therefore you will realise how important it is to include vegetables and fruits, w hich are rich source of these two

vitamins, in our daily menu of cereals and dal. Most of the vegetablres and fruits, as you will observe

from Table 1.1, are low in calories. Oils, fats and sugars are mainly sources of calories. Thus you get an idea of the contribution of various foods by studying Table 1.1. Most of the analytical work on Indian foods was carried out in various l aboratories under the auspices of Indian Council of Medical Research. A compilation of results is published as the Nutritive Value of Indian Foods, by the Indian Council of Medical Research (ICMR). A number of new var ieties of food with high contents of certain nutrients, have been developed at research centres under the

auspices of the Indian Council of Agricultural Research. You get a number of these foods in the market

and use these in your dietary. The nutritive value of these new varieties of foods need to be include

d in

the book on Nutritive Value of Indian Foods. There are two International Food Value tables published by

Foods, Nutrition and Health99999

the Food & Agriculture Organisation (FAO) (please refer to these books, which are listed in Further

Reading at the end of this book).

It is good to remember that the nutritive value of natural foods does no t vary a great deal for a

particular variety of the same food from one country to another. But there is a great variation in the

composition of prepared foods such as bread, biscuits, cakes, etc., due to variation in recipes and the basic ingredients used from one region to another.

Table 1.1: Food Composition at a Glance

1 (Approx. Group Values per 100 g. E.P.)

Vit. A Vit. CMinerals

CaloriesProtein (g) (mcg.)(mg) & Vit.

B-Comp

Cereals-rice,10340 7 to 12 - - Some

Wheat, bajra,

Jowar

Dals, legumes10340 17 to 25 - - Some

2

Milk8570 3 48 - Some

3

Eggs75170 13 960 - Some

Meat, fish, poultry 75100-190 18Some - Some

Leafy &9020 21,800 30Some

Orange-yellow

Vegetables &

Fruits

Fruits-Vit. C-rich 8550 1 Some 50 Some

Other vegetables 9030 2Some Some Some

Other Fruits85501 Some Some Some

Roots & tubers60-85 50-100 1Some Some Some

Oils & Fats0900 - 750

4 - -

Sugar, jaggery 0 400 - - - -

1. Please refer to Appendix F for individual composition of foods.

2. Rich source of iron and B-complex.

3. Rich source of calcium and riboflavin.

4. Ghee contains this amount and vanaspati is fortified to provide this amo

unt of Vit. A.

Food Exchange Lists

In 1950, the American Diabetes Association and the American Dietetic Ass ociation collaboratively developed a system of food lists to help diabetic patients to select foo ds in their diets. Similar food lists

FoodsMoisture

1010101010Fundamentals of Foods, Nutrition and Diet Therapy

were prepared in other countries to help diabetics to choose their foods . In India also food lists were prepared based on the foods available and our meal pattern. Since India is a large country there are three major agencies that have evolved their food exchange lists. These agencies are dietetic departments of major regional hospitals, the Home Science colleges, which train diet etic students, and the dietetics department of the National Institute of Nutrition. These are presented i n Appendix B. Each of the list includes a group of foods, which supply about the same calories in the portion indicated. Each food choice within a list is called an exchange. It repr esents an amount of food that has about the same macronutrient value as other foods in the same group. The exchange lists are very useful tools in diet planning in hospitals a nd in personal diet management in the home.

Nutrient Density

It refers to the quantity of one or more nutrients supplied by a food in reference to its calorie content. For example, if one compares the protein content of isocaloric portions of dal, bread and milk, one can see that dal has the highest nutrient density for protein, milk next and bread the l east.

CaloriesProtein

Dal855.5

Milk854.0

Bread852.4

Thus nutrient density is an important aspect to be considered in selecti on of foods, especially in diets of children, pregnant women, nursing mothers and in therapeutic di ets for patients.

How will you be a responsible nutrition student?

First, use what you learn in nutrition and benefit yourself. It will hel p you to look better, feel better and work effectively. By practising what you learn, you will set a good example for others. Secondly take care of nutrition of your family. Help the children in the family to develop good food habits, the adults to make good food choices in eating and seniors to meet their changed needs. Thirdly, as a professional you will be able to help people, who seek your advic e, to make appropriate changes to improve their food choices to improve their well-being. Lastly, as a member of the community, you can influence the choice of foods served in social events.

Study Questions

1. Explain the terms nutrition, nutrients, nutritional care, optimum nutrit

ion and undernutrition.

2. How is food related to health?

3. Discuss the physiological functions of food.

4. "Food is also used to satisfy social and psychological needs". Exp

lain this statement by giving examples.

5. What do you understand by adequate nutrition?

6. Explain the concept of exchange lists.

7. What is nutrient density?

8. How will you use your knowledge of nutrition?

CCCCCHAPTERHAPTERHAPTERHAPTERHAPTER 2 2 2 2 2

Digestion, Absorption and Utilisation of

Nutrients

Introduction

Basic Concepts of Biology and Chemistry

Digestion and Absorption of Nutrients

Carbohydrates

Proteins

Lipids

Other Nutrients

Factors Affecting Digestion

Role of Large Intestine

Utilisation of Nutrients in the Body

Glycolysis

Tricarboxylic Acid Cycle (TCA)

Beta-oxidation of Fatty Acids

Amino Acids

Deamination

Transamination

Urea formation

Study Questions

Introduction

OUR BODY is synthesised from the food we eat. It is made of a complex structure of cells, tissues and organs. How does this change from food to our body structure occur? All the changes that occur in the food from the time we eat it, to its use in the body and di scarding of the waste matter are known as metabolism. One can describe metabolism of each nutrient separa tely to ensure ease of understanding. But actually it occurs in a correlated systematic manner. Basic concepts of biology and chemistry need to be understood in the stu dy of nutrition. Let us review these.

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Concepts in Biology: The study of nutrition begins with the cell, the basic unit of our body . All the nutrition processes, which we refer to as metabolism, take place in the cell. Metabolism includes both anabolism and catabolism. Anabolism involves synthesis of compounds needed for use in the body. Breakdown of complex substances to simpler ones is known as catabolism . Thus cells are able to take up nutrients, synthesise substances they need and eliminate wast es. The energy release and its utilisation occurs in the cell. There are many kinds of cells in the body, each type specialises in carrying out certain functions required by the body. Cells are grouped together to form a tissue. Muscle, nerve, epithelial a nd connective tissue are examples of various tissues. Two or more tissues are combined to form an organ, which carries out a specific function. Heart, lungs and kidneys are examples of organs. Cells are made up of several parts. Each part has an appropriate structu re and a specific function. Two main parts of the cell are the nucleus and the protoplasm, which surr ounds the nucleus and is called cytoplasm (Figure 2.1). Figure 2.1: Diagram of a typical cell based on electron micrographs. (Adapted from "The Living Cell" by J. Bracket. Copyright © 1961 by Scientific American, Inc. All rights res erved.) Digestion, Absorption and Utilisation of Nutrients1313131313 The nucleus controls the functions of the cell; the metabolic activities of the cell are carried out by the cytoplasm. The deoxyribonucleic acid (DNA) in the cell nucleus contains the pattern for each of the different

proteins in the body. The ribonucleic acid (RNA) directs the actual protein synthesis in the ribosomes,

using the information stored in the DNA. This process, which is directed and controlled by DNA, is the key to nutrition. All the components, which form nutrients, come from food. Our genes dete rmine the nutrients that can be synthesised in the body and those which need to be provided preformed in the food. The small channels in the cytoplasm, called endoplasmic reticulum, trans port nutrients and their metabolites throughout the cytoplasm. The enzymes, which function in met abolism, are found in the membranes surrounding the channels. The mitochondria and lysosomes are also present in the cytoplasm. Mitochondria release the energy provided by the carbohydrates, fats and proteins and transfer it to a n energy acceptor (ATP). The ATP transfers the energy as needed wherever work is being done. Therefore the mitochondria are known as the 'power plants' of the cell. Lysosomes contain enzymes, which function in the breakdown of proteins an d other compounds. Lysosomes help to digest foreign matter that may have entered the cell an d thus protect the body from their harmful effects. Chemistry: Our body and the food that nourishes it are made of chemical elements.

Therefore

knowledge of the chemical elements and their behaviour is basic to the s tudy of nutrition. Elements: Elements are fundamental units of matter, which have characteristic properties. The Periodic Table contains all the elements. Of the 106 known elements, 92 occur in n ature, the rest are of synthetic origin. Examples of elements are oxygen, carbon and iron. Atoms: The smallest constituent part of an element is an atom. There are small er particles, which are parts of an atom. The proton (+vely charged) and neutron (uncharged) particle are located in the nucleus of the atom. In the shells (orbits) around the nucleus, the electron, a negatively charged particle is found. Atoms of one element can combine with atoms of another element to form a compound. For example, water contains two atoms of hydrogen and one of oxygen (H 2 O). Molecule: Chemical elements exist as molecules, a basic unit, which can be made u p of atoms of one or more elements. Thus we have a molecule of oxygen (O 2 ) with two atoms of oxygen, while sodium chloride (NaCl) contains one atom of sodium and one atom of chl orine. Ion: An electrically charged atom, group or molecule is called an ion. A positively, charged ion is called a cation, and a negatively charged one an anion. K + is an example, of a positive atom and Cl - is a negative atom; potassium chloride (KCl) is a neutral compound. Isotopes: Some elements have isotopes, that is they exist in two forms, with two different atomic weights; for example, the atomic weight of carbon is 12, its isot ope has an atomic weight of 14 ( 14

C). The

14

C isotope of carbon is naturally radioactive.

Acids, Bases, pH: A chemical compound, which when dissolved in water yields hydrogen ions (H + ), is an acid. There are two types of acids, inorganic acids and organi c acids. Hydrochloric, sulfuric and phosphoric are inorganic acids. Organic or carboxylic acids contain one or more carboxyl groups (COOH) in their molecule. Fatty acids and amino acids are important or ganic acids in nutrition. Hydrogen ion concentration is the amount of hydrogen ion (H + ) per unit volume of an aqueous (water) solution. It is referred to as pH.

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Some Chemical Reactions: Some chemical reactions, which occur in biological systems, are listed here.

1. Salt formation: When an acid and a base react, salt is one of the products formed.

HCl + KOH

KCl + H 2 O acid + base
salt + water

2. Ester formation: When an organic acid reacts with an alcohol, an ester is formed. When a

fatty acid reacts with glycerol, a fat (an ester) is formed.

3. Oxidation-reduction: Oxidation is always accompanied by a reduction, in which electrons

are transferred from one atom to another. The atom that gives up the electron is oxidised, the one that receives the electron is reduced. Thus ferric ion is reduced by vitamin C to ferrous form. Fe +++ + reduced form of vit. C
Fe ++ + oxidised form of vitamin C

4. Hydrolysis: The reaction of water with another compound, with uptake of a molecule

of water, is known as hydrolysis. The breakdown of carbohydrates and fats in dig estion is an example of hydrolysis.

5. Phosphorylation: is a chemical reaction in which a phosphate group is introduced into an

organic compound. The formation of ATP from ADP is an example of this reaction.

6. Synthesis: When a complex substance is formed from simpler substances by a series

of reactions it is known as synthesis. Synthesis of proteins from amino aci ds is a good example of this reaction.

7. Degradation: Complex substances are gradually broken down to simpler ones. The

breakdown of glycogen to glucose is an example of degradation reaction.

Use of Food in the Body

The use of food in the body involves three processes - digestion, abs orption and utilisation of nutrients in the body. Digestion is the process, which releases many nutrients in the forms the body can use, by breaking up food in the intestinal tract. Absorption is the process which carries these nutrients into the circulation syste m and delivers them to the cell. Utilisation. Cell is the functional unit of life. Hence a large number of the chemic al reactions in the cell utilise the nutrients absorbed to produce materials needed for our existence. Mechanical processes involved in digestion include chewing of food, swallowing of food, chur ning action in the stomach and rhythmic contraction of the intestinal tract. Chewing of food reduces the food particles in size, mixes these with sal iva and dilutes it with water, so that it is easy to swallow. The food swallowed is mixed with enzymes and acid by the churning action in the stomach. Further the rhythmic contraction of the intestine, help to break the food into small particles and move the food mass forward through the dig estive tract (Figure 2.2). Chemical reactions in digestion process: The first reaction is hydrolysis or splitting with the help of water. Carbohydrates, fats and proteins break up with the addition of water i nto smaller molecules, which the tissues can use. Digestion, Absorption and Utilisation of Nutrients1515151515

Figure 2.2: The digestive tract.

The chemical reactions are accelerated by enzymes, which are secreted in the mouth, stomach and small intestine. Enzymes are living catalysts, that increase the spe ed of biological reactions, without being a part of the compound formed. The enzymatic reactions, which take place in the digestion process, are presented in Table 2.1. As you may know, enzymes are proteins by nature. Their name indicates substance on whic h they can act, for example, sucrase acts on sucrose. Enzymes are specific in their actions. An enzyme, which hydrolyses protein, will not act on starch. Each enzyme acts optim ally at a certain pH, e.g., amylase acts only in alkaline medium. Some enzymes need another group, known as a coenzyme, to be attached to it to aid their function. For example, B-vitamin serve as coenzymes in the reactions, wh ich release energy from glucose. In enzyme reactions, mineral elements are essential as cofactor s. Thus normal body metabolism is dependent on the presence of appropriate enzymes, coenzymes and cofac tors specific to each reaction. The digestive enzymes are only one group of a large number that are esse ntial to regulate body processes. Other enzymes are present in various tissues of the body and help in the utilisation of food that has been absorbed.

Carbohydrates

The digestion process begins with chewing the food in the mouth. The enz yme ptyalin (salivary amylase) starts the digestion of starch in the mouth. It hydrolyses sta rch to dextrins, isomaltose and maltose in neutral or alkaline pH in the mouth. The food tastes sweet du e to these products of hydrolysis. The activity of amylase continues in its movement from the mouth to the upper part of the stomach. But as soon as the food mass comes in contact with hydrochloric acid sec reted there, this action ceases. Very little digestion of carbohydrate occurs in the stomach as the pH is unfavourable.

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EnzymeOptimum pHSubstrate

The food mixed with gastric juice forms a semi-fluid mass called chyme. It takes about three to five hours to form chyme. Small portion of chyme are released through th e pyloric sphincter into the duodenum, the first part of the small intestine. The small intestine is so named because of the small diameter of its tube. It is about 20 f t long. Most of the digestive activity takes place in its three compartments nam ely the duodenum, the jejunum and ileum. Carbohydrate digestion occurs almost completely in the small intestine, mainly in the duodenum. Pancreatic amylase breaks starches into maltose and dextrins. The maltase from mucosal cells breaks down maltose to glucose. The brush border, on the surface of the epithelial cells lining the intestines, is the site of this enzyme action. The enzymes sucrase, lactase, maltase, and isomaltase, found on the outer cell membranes of the intestines, act on the sugars sucrose, lactose, maltose and isomalto se respectively. The monosaccharides formed - glucose, galactose and fructose - pass throug h the mucosal cell and via the capillary into the blood stream. These are carried to the liver by the p ortal vein.

Table 2.1: Enzymatic Reactions in Digestion

Place ofProducts of

ActionReaction

MouthSalivary amylase 7.0Cooked starchDextrins, maltose (Ptyalin)

Stomach Pepsin (protease) 2.0ProteinsPolypeptides

Rennin6.0-6.5Milk, casein Calcium caseinate

Lipase7.0Emulsified fatsFatty acids, glycerol

SmallPancreatic Juice

IntestineTrypsin (Protease)8.0-9.0ProteinsPolypeptides, some amino-acids

Lipase7.0FatsDi and mono-

glycerides, fatty acids, glycerol

Amylase7.1StarchMaltose

Intestinal Juice

Peptidases8.0Peptones,Amino acids

(Erepsin)Polypeptides

Sucrase5.0-7.0 SucroseGlucose, fructose

Maltose6.7-7.2MaltoseGlucose(2 molecules)

Lactase5.4-6.0 LactoseGlucose, galactose

1. pH is expressed as the negative logarithm of 'hydrogen ion concent

ration'. The pH of water is 7; the acids have pH less than 7 and alkalies have pH above 7. Some glucose is stored in the liver and muscles as glycogen, the rest is transported to tissues to be used for their activities. Fructose and galactose are converted to gl ucose in the liver. Digestion, Absorption and Utilisation of Nutrients1717171717 Cellulose, hemicellulose, lignin and other forms of carbohydrate, which are collectively known as fibre, are not split by human amylases. These are excreted in the faeces .

Proteins

Digestion of proteins starts in the stomach, which serves as a storehous e, where some protein hydrolysis begins. Milk is clotted by a special enzyme rennin and acid i s added. Gastric juice, which is secreted by the stomach, contains hydrochloric a cid, pepsin, rennin, mucin and other substances. Hydrochloric acid has several important functions: (1) it swells the p roteins, thus increasing their surface area for enzyme action, (2) it converts the inactive pepsinogen to the enzyme pepsin, (3) it provides the acid medium necessary for the action of pepsin, (4) it pr ovides acidic pH for solution of calcium and iron salts, (5) it reduces or destroys the activity of man y pathogens (harmful bacteria) present in the food. Pepsin is the only proteolytic enzyme, which is able to digest collagen, the main protein in connective tissue. Pepsin, a protease in the gastric juice, splits proteins to poly peptides. It also digests the milk curds formed by the enzyme rennin. The contribution of the stomach to th e total process of protein digestion is small, as most protein digestion occurs in the duodenum. As soon as the chyme enters the duodenum, it stimulates the intestinal m ucosa to release an enzyme enterokinase, which converts inactive trypsinogen into active trypsin. Trypsin activates other proteolytic enzymes - chymotrypsin and carboxypolypeptidases. These enzymes breakdown intact protein and with the help of peptidases continue the breakdown un til small polypeptides and amino acids are formed. The last phase of protein digestion also occurs in brush border, in which peptidases hydrolyse di- and tri-peptides into constituent amino acids.

But some intact peptides do

escape hydrolysis and enter the portal circulation. The amino acids released are absorbed via distinct active transport syst ems. Absorbed amino acids and peptides are transported via the portal vein to the liver to b e released into the general circulation. By the time it reaches the jejunum, almost all the protein is absorbed.

Only one per cent of

digested protein is excreted in the faeces. Some amino acids, which remain in the epithelial cells, are used in the synthesis of new cells and intestinal enzymes. The endogenous protein released internally (by brea king down of epithelial cells and intestinal secretions) is digested and absorbed from the small intestin e along with that ingested in the diet.

Lipids

The stomach lipase does act on emulsified fats in milk, cream, butter an d egg yolk, but most of the hydrolysis of fats takes place in the small intestine. As soon as the food enters the duodenum, the gall bladder releases some bile and the pancreas secrete enzyme-rich juices. These fluids enter the duodenum through a co mmon duct. Bile is a secretion composed of bile acids, bile pigments, cholesterol, lecithin and many ot her compounds. About a liter of bile is secreted daily due to the stimulus of food in the duodenum. The bile, which is manufactured by the liver, helps to •emulsify fat particles, which increases surface area and improves enzyme action. •neutralise the acid chyme and provide the alkaline pH necessary for the action of the intestinalenzymes.

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The pancreatic juices contain trypsin, lipase and amylase, which act upo n all components of foods. Thus proteins are broken down by trypsin into amino acids. Large sugar molecules are changed to simple sugars by action of amylase and lipase acts on glycerides and reduces these to fatty acid and glycerol. The products of fat digestion inhibit the digestion process. The digeste d meal must be removed from the duodenum over a period of time, to permit digestion to proceed. Hence a fatty meal may remain up to four hours or more in the stomach. Thus there is a feeling of satiation after a fatty meal. Micelles, a combination of free fatty acids, monoglycerides and bile salts, facil itate the passage of lipids through the aqueous environment of the intestinal lumen to the brush border. The bile salts are then released from the lipid complex and return 3 to 15 times to carry m ore lipids across the membrane, depending on the amount of food ingested. The fatty acids and monoglycerides are reassembled into new triglyceride s in the mucosal cell.

Chylomicrons

are formed by surrounding a combination of triglycerides, cholesterol, and phospholipids with a beta-lipoprotein coat. Chylomicrons are transported by lymphatic vessels into the blood stream and further on to the liver. The triglycerides are transported to adipose tissue for metabolism and storage. Cholesterol is hydrolysed by pancreatic cholesterol esterase from ester form and absorbed in the same manner as lipids. The fat soluble vitamins A, D, E and K are also absorbed in a similar fa shion. Some forms of the vitamins A, E and K and carotene do not need bile acids for their absorp tion. Normally 97 per cent of ingested fat is absorbed into lymph vessels. Fat ty acids of 10 carbons or

less, due to shorter length and increased solubility, do not need bile salts and micelle formation for their

absorption and can be absorbed directly into the mucosal cell. From the mucosal cell these acids go directly via the portal vein to the liver, without esterification. This is clinically a very useful quality of medium-chain fatty acids. Th ose patients, who cannot metabolise usual long-chain fatty acids in dietary fat, due to lack of b ile salts or some other problem, can be fed triglycerides of medium-chain fatty acids (C 8 and C 10 ). These can be utilised directly. The digestive process is also aided by friendly bacteria which live in t he intestinal tract. They help in hydrolysing food and manufacture some of the nutrients (e.g., vitami n K, folic acid and some B- complex vitamins).

Other Nutrients

Vitamins and water pass unchanged from the small intestine into blood by passive diffusion. Mineral absorption, which is more complex, occurs in three stages. In th e first stage, chemical reactions occur in the stomach and intestines, which are affected by the pH of the mix. In the second stage, these are carried across the membrane into intestinal mucosal cel l. In the third stage, minerals are transported into the blood stream or are bound within the cell. Important interactions occur between minerals in the gastrointestinal tr act, which affect the amount that is absorbed. There is simultaneous absorption of vitamins, minerals and fluids throug h the intestinal mucosa. About 8 liters of fluids move to and from across the membrane of the gut to keep the nutrients in solution. The current understanding of the sites and routes of absorptio n of nutrients is depicted in

Figure 2.3.

Digestion, Absorption and Utilisation of Nutrients1919191919 Figure 2.3: Gastrointestinal tract-sites of secretions and absorption.

Factors Affecting Digestion

Several factors affect the digestion process. These include:

1. Psychological factors, which play an important role in food acceptance,

ingestion and digestion. If one likes the sight, smell and taste of food, it increases secretions of saliva, stomach juices and motility of gastrointestinal tract. Even the thought of food increas es these secretions. On the other hand, if these are not liked, it depresses the secretions.

2. Generally well cooked foods are more digestible than raw foods.

3. Sound teeth to bite, cut and tear food into smaller pieces helps digesti

on, absence of molars, incisors and/or canine teeth affects the person's ability to grind the food adversely and hence digestion is poor.

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4. Sufficient supply of water to dilute the food, permits effective movemen

t through the digestive tract and increases the surface area for efficient enzymatic breakdown.

5. Sufficient bulk (fibre) helps to maintain normal forward movement.

6. Normal secretion of enzymes, bile, hydrochloric acid aids the chemical s

plitting of food components to small absorbable units.

7. The presence of friendly bacteria in the intestinal tract helps digestio

n.

8. Relaxation of body permits the processes involved in digestion to occur

normally. Any psychological stress, which makes a person tense, interferes with the di gestive process by upsetting the occurrence of the normal secretions mentioned above.

Role of Large Intestine

Large intestine consists of cecum, colon, rectum and the canal and is ab out 5 feet long. About 500 to 1,000 ml of water in the chyme enters the colon each day. Most of it is absorbed and only 50 to 200 ml is excreted in the faeces. Since the colonic conte nts move forward very slowly, most of the nutritionally valuable matter is absorbed. The large amounts of mucus secretion by the mucosa of large intestine pr otects the intestinal walls from the adverse effects of bacterial and other action. The mucus also helps to hold the faeces together. The neutralisation of acidic products of bacterial action is ensured b y the action of bicarbonate ions secreted in exchange for absorbed chloride ions.

Absorption and Transportation

Absorption is the process of sucking up the nutrients in the body. Most of the absorption takes place in the small intestine. Absorption of food from the digestive trac t into the blood and lymph takes place after the digested food is moved forward by peristaltic waves (mu scular contraction and relaxation) into the small intestine. These peristaltic waves push the food against the absorbing surface of the intestinal wall. The intestinal wall is lined with four to five million tiny finger-like projections called villi (see Figure 2.4.). The cells which cover villi permit the absorption o f final products of digestion -

small molecules of sugars, amino acids and fat products and water, into the vessels that carry away the

blood and lymph. The large area of these villi promote efficient absorpt ion of all nutrients. Ordinarily a great deal of time is permitted for the absorptive process because the d igested material must traverse the entire length of the intestine a distance of about 20 ft. Besides the shape and size of intestinal wall, other factors affect abso rption of the digested food. For example, bile favours absorption of fats, calcium can be absorbed in the presence of vitamin D and vitamin B 12 can be absorbed only in the presence of an intrinsic factor in the gast ric juice. The intestine, which is a semipermeable membrane, is highly selective an d permits the passage of nutrients only under certain special conditions. Thus absorption is affe cted by concentrations of the nutrients in the blood. A high concentration in the blood stream may all ow less absorption of vitamins and minerals than would be permitted at a time when the blood level of t hese nutrients is low. The sugars, amino acids, water-soluble vitamins, minerals salts and poss ibly some of the fat products are absorbed directly into the blood stream through the intesti nal wall and are carried to the liver. The remainder of the end products of fat digestion and the fat soluble vitamins enter into the lymph system. Digestion, Absorption and Utilisation of Nutrients2121212121 Individuals in normal health may vary in their capacity to absorb even f ully hydrolysed foods. The recommended daily dietary allowances and the daily food guide provide qu antities of nutrients generally sufficient to cover the needs of individuals with normal variations in c apacities to digest and absorb nutrients. Figure 2.4: Intestinal wall - mucosal folds, villi and microvilli.

Utilisation of Nutrients in the Body

Metabolism of all nutrients occurs within the cells of our body. The final oxidation of nutrients into carbon dioxide and water occurs in the mitochondria. About 40 per c ent of the energy released is used to synthesise ATP, the remainder is released as heat. The ATP formed provides energy for the synthetic reactions in the cell. Fatty acid breakdown also occurs in the mitochondria. The individual cells die and are replac ed by new cells formed as part of normal life processes. Glycolysis: Within the tissue cells, the first phase of glucose breakdown occurs in t he cytoplasm. This anaerobic stage of carbohydrate breakdown is known as glycolysis, in which glycogen or glucose is converted to pyruvic acid. This is schematically presented in Figure 2.5. Tricarboxylic Acid Cycle (TCA) or Citric Acid Cycle: The intermediate compounds of carbohydrate, fat and protein metabolism are metabolised by a common fin al oxidative pathway. This pathway is referred to as tricarboxylic acid cycle (TCA) or Citric aci d cycle or Krebs cycle. The name tricarboxylic indicates that it involves acids with three carboxyl ( -

COOH) groups; it is also called

Krebs cycle, as Krebs was the first scientist to have worked it out. In the mitochondria, pyruvic acid is converted to acetyl coenzyme A CoA before entering the citric acid cycle.This complex reaction needs five enzymes and four coen zymes, four of which contain four B vitamins (pantothenic acid, thiamin, niacin and riboflavin). Th e pantothenic acid is a part of coA structure. The two carbon acetic acid combines with four carbon oxaloace tic acid forming citric acid (6-C acid), with release of coenzyme A. This is the starting point of the citric acid cycle, which is

Enlarged villi, showing

"brush border" of microvilli

Muscularis mucosae

Mucosal folds

(valvulae conniventes) Villi

Circular muscle layer

Longitudinal muscle

Outer serosa

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depicted in Figure 2.5. In this figure all steps at which hydrogen atoms , carbon dioxide or ATP are formed are shown; a few